Roller bearings in crankshaft?

[Designori..]

Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?

Des.

[Simon Barr]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?
Des.

I guess it's cos you'd then need to have a pressed together crank like some
bike engines and that's probably more expensive. They are also prone to
twisting, a mate of mine twisted his GS1000 crank when he missed gear once.

Also I reckon that a plain bearing would have a much larger surface area to
spread the load across.

[JB]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?
Des.

On many motorcycle engine cranks, roller bearings are used fairly often. The
upside is longevity (if the oil is kept clean), but there are many downsides
as far as the motor manufacturers are concerned, mostly related to cost.
Most cranks are forged from one piece of steel then the shell-bearing
journals are ground to size. This is a cost effective and well understood
technology and the machinery is well proven. Also, if a shell bearing gives
trouble, it is very straightforwward to regrind the journal and replace with
oversize shells. For roller bearings, the crank has to be produced from
separate sections then pressed together very accurately. To replace a roller
bearing on one of the inner journals of a pressed-up crank, it can get very
expensive and is often cheaper to replace the whole crank assembly.

JB

[Tony Bond (UncleFista)]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?
Des.

The

[David A Stocks]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?
Des.

Because you couldn't do it with a single-piece crankshaft, which creates a
whole load of other problems - especially in a high speed engine - and is
expensive to manufacture. I did once come across a vintage single cylinder
tractor engine with roller bearings and a built-up crankshaft - max speed
around 1000 revs/min.

D A Stocks

[Peter Hill]

On Tue, 14 Dec 2004 18:09:59 -0000, "David A Stocks"

Post by Designori..

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile

environment

Post by Designori..
any bearing set on the engine has to endure, how come roller bearings

aren't

Post by Designori..
used for that application?
Des.

Because you couldn't do it with a single-piece crankshaft, which creates a
whole load of other problems - especially in a high speed engine - and is
expensive to manufacture. I did once come across a vintage single cylinder
tractor engine with roller bearings and a built-up crankshaft - max speed
around 1000 revs/min.

Purely down to cost and "we've always done like that".

You can get split race (and cage) ball, roller and needle bearings
that would allow a one piece crank but they are very expensive.

Some of the highest revving engines in the world run on roller
bearings. All modern 2 strokes use roller and ball bearing as a shell
can't cope with next to no lube. 12,000 rpm was not uncommon and some
60's racers ran 18,000rpm. Model aero engines run on ball bearings -
50-60,000rpm. Turbos either use a floating bearing (shaft runs in a
bush which runs in the housing so splitting the rpm between two plain
bearings) or they use a ball bearing, running at 100,000 rpm and up.
Jet engines run HP spool 12,000rpm on a ball bearing at the front and
a roller at the rear. HP turbine on an 80K lb thrust engine produces
77,000bhp.

Back in the mid 70's a car mag set out to destroy a poor little Honda
CB125S. They ran it out of oil and it kept going, so they stuck it on
it's stand and held the throttle wide open in neutral. Con ron
ventilated the crankcase at 18,000rpm. Roller bearing crank. Red
line was 10,000rpm but I had mine out the other side to 12,000rpm in
top.

Look up Tatra they used roller bearing cranks for cars and trucks.

[Guy King]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?

I'd have thought the small end was the most hostile - higher
temperatures and a back and forth action which must be harder to
lubricate than rotary.

Anyway - since the oil pump doesn't seem to have much trouble keeping
the crankshaft afloat so there's no metal to metal contact, why change
it.

They're cheap, simple and they work. And quiet, of course.

[Peter Hill]

Post by Guy King

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?

I'd have thought the small end was the most hostile - higher
temperatures and a back and forth action which must be harder to
lubricate than rotary.

Which is why all the Japanese 2 strokes and most others since the mid
60's had needle roller little ends. (Well not small stuff like chain
saws and mowers)

As the little end motion is only about 35 degrees total (~1/10th of
motion on big end or mains) on a small ~20mm dia pin (~2/5ths size of
big end or mains) the velocity of pin surface to piston or rod bore is
much slower. At max rpm it's comparable to running at tickover on the
mains. Next to no wear is caused in a well lubed oil mist such as you
find in a fully warm 4 stroke. Just go steady until you have hot oil
to make that nice lubricating mist.

Excessive over revving can make little ends go oval as the inertia
load of the piston at TDC stretches the little end.

[Peter Hill]

On Tue, 14 Dec 2004 23:54:58 +0000, Peter Hill

Post by Peter Hill
Excessive over revving can make little ends go oval as the inertia
load of the piston at TDC stretches the little end.

Forgot to mention it's almost completely unloaded at TDC during
ignition as the gas pressure on the piston balances the piston
inertia. So highest load is during TDC during valve overlap and BDC
on both strokes. Assuming you don't run into detonation that's just
nasty for a whole lot of bits as the piston tries to make the engine
go backwards while the whole inertia of the vehicle drives it
forwards.

[s***@despammed.com]

On Tue, 14 Dec 2004 23:54:58 +0000, Peter Hill

Post by Peter Hill
Which is why all the Japanese 2 strokes and most others since the mid
60's had needle roller little ends. (Well not small stuff like chain
saws and mowers)

All my chainsaws do have needle roller little ends.

AJH

[Mike G]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?

They are. They are quite common in m/c engines, but in fact size for size,
plain bearings are capable of taking higher loads than roller bearings.
Mike.

[SimonJ]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?

Because a roller bearing needs to slid over the end of a shaft, and you cant
get to the end of a crank journal, because the webs are in the way!

Some engines do have roller bearing cranks (very rare) not sure how they do
it, will have to be a cranks that can be split, or some sort of splitable
roller bearing. Either way would be complicated to get right.

[Robin Graham]

I'm guessing here, but I can think of several reasons.

1. White metal bearings seem to work well and are not expensive.

2. The assembly would be a (horrendous) problem. Some single cylinder
motorcycle engines have roller bearings but you can split the crankpin away
from one half of the crank web, put on the bearing and put the crankpin and
its nut back. Then you have to align the journals at the ends of the crank
on a lathe-type machine. I suppose you could make roller bearings where the
races were in two halves but the thought makes me go weak at the knees.

3. If a roller bearing departed this life it's likely that it would happen
very quickly, whereas plain bearings generally give some indication that
they are tired.

Anyone got any more ideas?

Rob Graham

[Chris Street]

Post by Robin Graham
I'm guessing here, but I can think of several reasons.
1. White metal bearings seem to work well and are not expensive.
2. The assembly would be a (horrendous) problem. Some single cylinder
motorcycle engines have roller bearings but you can split the crankpin away
from one half of the crank web, put on the bearing and put the crankpin and
its nut back. Then you have to align the journals at the ends of the crank
on a lathe-type machine. I suppose you could make roller bearings where the
races were in two halves but the thought makes me go weak at the knees.
3. If a roller bearing departed this life it's likely that it would happen
very quickly, whereas plain bearings generally give some indication that
they are tired.

My bike mechanic races 1960's vintage bikes with a pressed up roller cam.
He reckons that from starting to hear a roller go to the event termed as
Apocalypse Now he has about five seconds and coasting the engine down from
18,000rpm (a four stroker with DOHC at 18k - scarey!) takes three seconds!

[Biggles]

Post by Robin Graham
I'm guessing here, but I can think of several reasons.
1. White metal bearings seem to work well and are not expensive.
2. The assembly would be a (horrendous) problem. Some single cylinder
motorcycle engines have roller bearings but you can split the crankpin away
from one half of the crank web, put on the bearing and put the crankpin and
its nut back. Then you have to align the journals at the ends of the crank
on a lathe-type machine. I suppose you could make roller bearings where the
races were in two halves but the thought makes me go weak at the knees.
3. If a roller bearing departed this life it's likely that it would happen
very quickly, whereas plain bearings generally give some indication that
they are tired.
Anyone got any more ideas?
Rob Graham

Can't be that horrendous a problem. I had a 1966 Saab 3-cyl two-stroke with
roller bearing crank. I never had the crank apart so can't comment on
construction bit I think the bearings were conventional design (with very
large balls). Come to think of it, aren't most/all two-strokes roller
bearing? I guess in a two-stroke, because you don't have a camshaft etc to
lubricate, the saving on oil pump and machining oil passages may offset the
extra cost of fabricating the crank assembly? The distributor drive was in a
sealed chamber lubricated by grease.

Biggles

[Peter Hill]

On Tue, 14 Dec 2004 23:37:25 -0000, "Biggles"

Post by Biggles

Post by Robin Graham
I'm guessing here, but I can think of several reasons.
1. White metal bearings seem to work well and are not expensive.
2. The assembly would be a (horrendous) problem. Some single cylinder
motorcycle engines have roller bearings but you can split the crankpin away
from one half of the crank web, put on the bearing and put the crankpin and
its nut back. Then you have to align the journals at the ends of the crank
on a lathe-type machine. I suppose you could make roller bearings where the
races were in two halves but the thought makes me go weak at the knees.
3. If a roller bearing departed this life it's likely that it would happen
very quickly, whereas plain bearings generally give some indication that
they are tired.
Anyone got any more ideas?
Rob Graham

Can't be that horrendous a problem. I had a 1966 Saab 3-cyl two-stroke with
roller bearing crank. I never had the crank apart so can't comment on
construction bit I think the bearings were conventional design (with very
large balls). Come to think of it, aren't most/all two-strokes roller
bearing? I guess in a two-stroke, because you don't have a camshaft etc to
lubricate, the saving on oil pump and machining oil passages may offset the
extra cost of fabricating the crank assembly? The distributor drive was in a
sealed chamber lubricated by grease.

Can not run a thin wall shell bearing on total loss lubrication and
having to seal every main and big end bearing would be hopeless for
seal drag. The need for a seal between each crankcase chamber also
gave high drag. On racing 2 strokes seal drag was so important that
they used expensive non contact labyrinth seals. Big truck 2 stroke
diesels have an external pump to scavenge them so they can use a
journal bearing crank and big ends with just a seal at each end of the
crank.

Also roller bearings have much less drag giving a higher mechanical
efficiency. This is why they tended to be used on motorcycle engines
where power output is the prime consideration. Simple to install with
virtually zero maintenance - witness all those wheels bearing we are
running around on. No sump, no pump, no filter and no oil changes.

[Biggles]

Post by Robin Graham
I'm guessing here, but I can think of several reasons.
1. White metal bearings seem to work well and are not expensive.
2. The assembly would be a (horrendous) problem. Some single cylinder
motorcycle engines have roller bearings but you can split the crankpin away
from one half of the crank web, put on the bearing and put the crankpin and
its nut back. Then you have to align the journals at the ends of the crank
on a lathe-type machine. I suppose you could make roller bearings where the
races were in two halves but the thought makes me go weak at the knees.
3. If a roller bearing departed this life it's likely that it would happen
very quickly, whereas plain bearings generally give some indication that
they are tired.
Anyone got any more ideas?
Rob Graham

Can't be that horrendous a problem. I had a 1966 Saab 3-cyl two-stroke with
roller bearing crank. I never had the crank apart so can't comment on
construction bit I think the bearings were conventional design (with very
large balls). Come to think of it, aren't most/all two-strokes roller
bearing? I guess in a two-stroke, because you don't have a camshaft etc to
lubricate, the saving on oil pump and machining oil passages may offset the
extra cost of fabricating the crank assembly? The distributor drive was in a
sealed chamber lubricated by grease.

Biggles

[m***@ubht.swest.nhs.uk]

Apaprt from all the other replies IIRC the Great Western railway tried
roller bearings on steam engine cranks and coupling rods and found that
they were no more efficient than plain bearings and cost far more to
produce and maintain. Of course the application is slightly different
but the pressures involved must be similar.

[FatSod]

Post by Designori..
Since the pressures are pretty fierce and it's the most hostile environment
any bearing set on the engine has to endure, how come roller bearings aren't
used for that application?
Des.

The pressures are too fierce for rollers to endure. Also you have to
understand that the crankshaft and conrods don't actually sit on bearing
shells, they sit on a thin layer of oil between the bearing journel diameter
and bearing shell it'self.
You also have to remember that the shell bearings used as sacrifice should
things fail or wear out. If a roller bearing was used then I suggest that
should
it fail, any bits and peices flying around in the sump could do hell of a
lot of damage elsewhere rending the crankshaft or even engine to be
scrapped.

I used to think our company made big diesels untill I saw this......

http://www.bath.ac.uk/~ccsshb/12cyl/


-
Fatsod

[SimonJ]

Post by FatSod
http://www.bath.ac.uk/~ccsshb/12cyl/

Bloody Hell.